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Photon energies relative to movement, and relativistic mass Q's.

  1. Apr 27, 2009 #1
    Not sure if this should go here or in the relativity forum... let me know if I need to move it.

    I was reading something, and it said this:

    "A photon also has a rest mass, which is zero, even though a photon is always moving at the speed of light, and so is never at rest. But if an observer moves fast enough in the same direction as a photon, sees the photon as having less and less energy. By chasing the photon, it can be made to have as little mass-energy as you like. As you chase it faster and faster, the photon looks redder and redder, by doppler shift, and the energy of a very long-wavelength photon approaches zero as you approach the speed of light."

    Now, in my limited knowledge of relativity, light is always supposed to go the same speed, no matter what speed you are traveling, right? Are they trying to say something different here, or are they just saying that because the energy is related to the wavelength, the dopler effect can basically make the wavelength go to infinity and cause the energy to go to 0?

    If that is true, doesn't that present and interesting problem of the energy of light/photons being completely relative, even if they are still moving (a moving photon particle still moving at the speed of light would have effectively 0 energy, which doesn't make sense to me)?

    I could be completely wrong here, but it was just something I was wondering.

    My other question is a little quicker. As something goes faster and faster, therefore gaining mass according to relativity, does the object become more dense? Bigger? etc... I have a thought about that, but if there is a concrete answer for it already, I would like to hear that.

    Thanks for clarifying.
  2. jcsd
  3. Apr 28, 2009 #2


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    Your beginning doesn't make sense. If you are traveling in the same direction as the photon ahead of you, you can't see it, since it is going away from you. You can see photons in the same direction coming from behind, which will be red shifted.
  4. Apr 28, 2009 #3
    Well, I don't think they mean literally "see" it. Either way, that whole paragraph is just confusing...
  5. Apr 29, 2009 #4
    They probably mean that the Doppler effect can make the wavelength arbitrarily long and frequency arbitrarily low. Energy of photons (or anything) is already relative. Even in Newtonian mechanics, energy depends on reference frame. KE of a massive particle already depends on velocity, which is THE archetypal relative measurement. PE of a particle even depends on where you put the *origin* of your reference frame, never mind its speed. And this is all without reference to anything Einsteinian.

    It's deceptive to talk about "Relativistic Mass," especially of a freely moving particle. It's not the inertia of the particle, nor anything having to do with Gmm/r^2. The object does not become bigger. It gets smaller due to length contraction. So its density increases.
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